Chemokines are a superfamily of chemoattractant cytokine proteins which primarily serve to regulate a variety of biological responses and promote the recruitment and migration of multiple lineages of leukocytes and lymphocytes to a body organ tissue. Chemokines are classified into four families according to the relative position of the first two cysteine residues in the protein. In one family, the first two cysteines are separated by one amino acid residue (the CXC chemokines) and in another family the first two cysteines are adjacent (the CC chemokines). In a third family, the first two cysteines are separated by three amino acids (CX3C chemokines). In a fourth family there is only one cysteine in the amino terminus (C chemokines).
The chemokine-receptor pair of CCL20/CCR6 regulates dendritic and T cell trafficking to sites of injury or infection through the establishment of concentration gradients in vivo (Liao et al., 1999). During the general inflammatory response, the ˜8 kDa chemokine is secreted by lymphoid cells into the extracellular matrix where it binds and activates its single, seven transmembrane G protein-coupled receptor (GPCR), CCR6 expressed on the surface of migratory cells (Baba et al., 1997).
Psoriasis is a chronic inflammatory skin disease affecting 2-3% of the global population (Kurd and Gelfand, 2009) characterized by the infiltration of Th17 cells to the skin in response to injury or autoantigens. T cells, including those with low expression of the gamma-delta T cell receptor, are recruited to the dermis and epidermis by a number of chemokines, including CCL20. Together with its receptor CCR6, CCL20 plays a critical role in the development of psoriasiform dermatitis in mouse models.
Current psoriasis treatments that target Th17 pathway cytokines and tumor necrosis factor-a (TNF-a) can be highly effective in treating humans with psoriasis (Langley et al., 2014). However, a neutralizing anti-CCL20 monoclonal antibody reduced psoriasis-like inflammation, suggesting that inhibition of CCL20/CCR6-mediated T cell recruitment may also be a useful therapeutic strategy (Mabuchi et al., 2011).
Chemokines engage their receptors via an extensive protein-protein interface that encompasses domains at the extracellular surface and a deep pocket within the transmembrane domain (the orthosteric site) that is occupied by the chemokine N-terminus (Kufareva et al., 2014). Native chemokines are typically full, balanced GPCR agonists that elicit characteristic cellular responses, including the release of intracellular calcium from the endoplasmic reticulum and cell migration. Chemokine receptor antagonists, such as small molecules, peptides or mutated chemokines, typically block GPCR signaling by preventing activation at the orthosteric site (Crump et al., 1997; Hassan et al., 2011; Rosenkilde et al., 2004).
Partial or biased agonists of chemokine receptors are characterized by a selective loss of efficacy, such as AOP-Rantes, which induces CCR5-mediated calcium signaling but lacks pro-migratory signaling and has altered receptor recycling (Rodriguez-Frade et al., 1999). Additionally, Zadir and colleagues showed that biased agonism of CCR7 by its two chemokine ligands, CCL21 and CCL19, results in differing patterns of GPCR kinase recruitment but no difference in Gi/o, signaling (Zidar et al., 2009). While N-terminal modifications can alter the pharmacologic properties of a chemokine by disrupting orthosteric site contacts, manipulation of the chemokine oligomeric state can also change its signaling profile in potentially useful ways.
Chemokines typically bind and activate their cognate GPCRs in the monomeric state, but self-association is important for binding to cell surface glycosaminoglycans and thus for in vivo chemokine function (Campanella et al., 2006). Members of the CXC and CC chemokine subfamilies generally adopt distinct quaternary structures that utilize either the β1 strand (CXC) or residues near the N-terminus (CC) for the dimer interface (Jansma et al., 2009). Changes in the oligomeric state of a chemokine can alter receptor binding, activation and downstream signaling. Dimerization of most CC chemokines occludes key receptor binding epitopes, rendering them nonfunctional as GPCR ligands (Tan et al., 2013). Some CXC chemokines, however, that have been modified to be constitutively dimeric are competent to bind their receptors and can function as partial agonists and inhibitors (Veldkamp et al., 2008). Among CC chemokines for which structures have been solved, CCL20 is unique in that it adopted a CXC-type dimer in two high-resolution crystal structures with no evidence for the canonical CC dimer (Hoover et al., 2002; Malik and Tack, 2006).
Accordingly, there is a current need for cost-effective pharmaceutical agents and treatment methods for treating psoriasis and other autoimmune or inflammation disorders.